EFFECT OF REWARD LEVEL ON INDIVIDUAL VARIABILITY IN DEMAND FEEDING-ACTIVITY AND GROWTH-RATE IN ARCTIC CHARR AND RAINBOW-TROUT

Groups of Arctic charr and rainbow trout were fed by using demand feed ers and their individual trigger actuations registered with a PIT-tag (Passive Integrated Transponder) system. Food was supplied at two rewa rd levels, low and high, to five replicate groups of each species for 21 to 27 days. The reward level was defined as the amount of food obta ined in response to a single trigger actuation. The effects of reward level on individual demand feeding activity and growth rate were asses sed. As a result of high demand feeding activity, the daily food ratio ns for trout were in excess of their needs at both reward levels. This can be ascribed to the fact that they compensated a low reward level by increasing their bite activity. In contrast, demand feeding activit y in charr did not differ significantly between reward levels. Instead , resulting food rations were limiting and excessive, at low and high reward levels, respectively. The variation in bite activity between in dividuals (measured as their proportional contribution to the total nu mber of trigger actuations within a group) for charr was significantly higher in the low-reward treatment than in the high-reward level. For trout, the variation in bite activity did not differ significantly be tween treatments. Differences in response to reward level are suggeste d to be due to the fact that the social hierarchy is weaker in trout t han in charr; i.e. the differences in bite activity between dominant a nd non-dominant individuals are smaller in trout. At both reward level s, the benefit of being dominant, measured in terms of growth rate was significant for charr but non-significant for trout.